Spine and Stomach deformation in Humanoïd character
I've managed to improve, but not solve, the deformation problem that I had by doing the following:

1: I slightly changed the mesh, making the stomach portrude more and making a nicer back bend. That enables me to bend the back less to achieve the illusion of a bend.

2: I divided the lower spine in 3 bones instead of 4 and shifted the whole spine and chest more to the centre of the mesh.

I think it looks significantly better and the side bends (wich I will not show) look pleasant.





I will still work on it and try to add strech bones. Maybe it will help. Or maybe, as someone has suggested, this is the limit of the blender rigging system and I will have to rely on shape keys.